VakarchukS. B. “Jackson-Type Inequalities With Generalized Modulus of Continuity and Exact Values of the $n$-Widths of the Classes of $(ψ,β)$-Differential Functions in $L_2$. I”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 68, no. 6, June 2016, pp. 723-45, https://umj.imath.kiev.ua/index.php/umj/article/view/1874.